Apparatus for fabricating a glass rod and method of same

ABSTRACT

The present invention provides an apparatus and a method for fabricating a glass rod capable of suppressing a diameter fluctuation of a drawn glass rod even in a case of a relatively large diameter reduction ratio between a glass preform and a glass rod, such as 60 to 95%. The diameter (D) of the glass preform for determining the ratio from a measured diameter data is acquired, the measured diameter data is obtained by measuring a diameter of the glass preform before being drawn along a longitudinal direction of the preform, and the feed speed (V1) is determined so that the feed speed (V1) varies depending on a fluctuation of the measured diameter data in the longitudinal direction.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application Nos.2010-200506, filed Sep. 8, 2010, and 2011-194106, filed Sep. 6, 2011,which are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for fabricating a glassrod and a method for the same, in particular, to an apparatus and methodfor fabricating a desirable diameter glass rod by feeding a relativelylarge diameter glass preform such as an optical fiber glass ingot into afurnace, heating the preform in the furnace and drawing the heatedpreform from the furnace.

2. Description of the Related Art

Japanese Patent Laid-Open No. 2006-193397 discloses a method forfabricating a desirable diameter glass rod by measuring a diameter of apreform during drawing at a region where a deformation (diameterreduction) is progressing and a diameter at a region where the diameterreduction is almost completed, and adjusting the feed speed and drawingspeed of the glass preform with respect to a furnace based on themeasured diameters.

The description of known art in Japanese Patent Laid-Open No. H11-011970(1999) discloses a method of pre-measuring a diameter of a glass preformalong a longitudinal direction of the glass preform, determining a ratiobetween a feed speed and a drawing speed of the preform, and fabricatinga constant diameter glass rod based on the based on the ratio.

Japanese Patent Laid-Open No. 2006-219331 discloses suppression ofdiameter fluctuations of a drawn glass rod caused by a shifting of areference position defining a feed speed and a drawing speed of a glasspreform as drawing process progresses, by changing the amount of a feedand the reference diameter position of the glass preform.

Conventionally, a ratio of a target diameter of a glass rod with respectto a diameter of a glass preform (referred to as a diameter reductionratio below) has been about 20% to 50% and relatively small.Accordingly, the control method of Japanese Patent Laid-Open No.2006-193397 could suppress the diameter fluctuation to a required level.Recently, however, a larger size optical fiber preform is required, anda glass rod having a relatively small diameter deformation, in which adiameter reduction ratio is about 60% to 95%, is required. For example,when a 160 mm to 170 mm diameter glass preform is drawn into a 150 mmdiameter glass rod, the diameter reduction ratio is 88% to 94%.

To implement the feed back control disclosed in Japanese PatentLaid-Open No. 2006-193397, it is necessary to measure a diameter ataround a position where a diameter is substantially reduced in adiameter decreasing region. In case of a relatively large diameterreduction ratio, however, a position where a diameter is substantiallyreduced is adjacent the heater in a furnace. Accordingly, it isdifficult to directly measure a diameter at this position. If thediameter used for the feedback control is measured at a location spacedfrom the heater to some extent so as to prevent an affection of theheater, the response of the feedback control could be lagged. Thus thefeed back control may not be appropriately implemented. As a result, alarge fluctuation can be generated in a drawn glass rod.

According to the method disclosed in Japanese Patent Laid-Open No.H11-011970 (1999), a relatively desirable diameter fluctuation value canbe obtained even at a diameter reduction ratio of 60% to 95% in the caseof a constantly stable diameter glass preform. The method, however, maycause an unacceptable diameter fluctuation (specifically, more than ±1%)at an end portion of a usable region in a drawn glass rod at the end ofa drawing process, when a glass preform has a relatively large diameterfluctuation in a longitudinal direction thereof.

The method disclosed in Japanese Patent Laid-Open No. 2006-219331 cansuppress a diameter fluctuation of a glass rod. In the method, however,a criterion for changing the reference diameter position is indefinite,and an unacceptable diameter fluctuation may be generated depending on acondition of a diameter fluctuation of a glass preform. In addition, inthe embodiment of the publication, a 130 mm diameter glass preform isdrawn into a 30 mm diameter glass rod, that is, a diameter reductionratio is considerably small, such as 23%. The publication fails todisclose a method for suppressing a diameter fluctuation of a drawnglass rod in case of a relatively large diameter ratio such as 60 to95%.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and a method for fabricatinga glass rod capable of suppressing a diameter fluctuation in a drawnglass rod even in case of a relatively large diameter reduction ratiobetween a glass preform and a glass rod, such as 60 to 95%.

A first aspect of the present invention provides a method of fabricatingglass rod that feeds a relatively large diameter glass preform into afurnace through a top portion of the furnace and draws the glass preformfrom the furnace through a bottom portion of the furnace so that therelatively large diameter glass preform is drawn into a relatively smalldiameter glass rod, includes the steps of:

controlling a feed speed (V1) and a drawing speed (V2) of the glasspreform so that a ratio (V2/V1) between the feed speed (V1) and thedrawing speed (V2) becomes a value ((D/d)²) determined based on adiameter (D) of the glass preform and a target diameter (d) of the glassrod;

acquiring the diameter (D) of the glass preform for determining theratio from a measured diameter data, the measured diameter data beingobtained by measuring a diameter of the glass preform before being drawnalong a longitudinal direction thereof; and

determining the feed speed (V1) so that the feed speed (V1) variesdepending on a fluctuation of the measured diameter data in thelongitudinal direction.

A second aspect of the present invention provides an apparatus forfabricating a glass rod, including:

a furnace;

a feeding mechanism configured to feed a relatively large diameter glasspreform into a furnace through a top portion of the furnace;

a drawing mechanism configured to draw the glass preform from thefurnace through a bottom portion of the furnace so that the relativelylarge diameter glass preform is drawn into a relatively small diameterglass rod;

a controller configured to control a feed speed (V1) of the glasspreform by the feeding mechanism and a drawing speed (V2) of the preformby the drawing mechanism so that a ratio (V2/V1) between the feed speed(V1) and the drawing speed (V2) becomes a value ((D/d)²) determinedbased on a diameter (D) of the glass preform and a target diameter (d)of the glass rod, wherein

the controller includes:

an acquisition unit configured to acquire the diameter (D) of the glasspreform for determining the ratio from a measured diameter data, themeasured diameter data being obtained by measuring a diameter of theglass preform before being drawn along a longitudinal direction thereof;and

a determination unit configured to determine the feed speed (V1) so thatthe feed speed (V1) varies depending on a fluctuation of the measureddiameter data in the longitudinal direction.

According to the present invention, a diameter for determining a ratiobetween a feed speed and a drawing speed is determined from pre-measureddiameter data, and the feed speed is adjusted depending on a diameterfluctuation of the preform. As a result, a diameter fluctuation of adrawn glass rod in a longitudinal direction of the glass rod can besuppressed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an apparatus for fabricating aglass rod according to an embodiment of the present invention;

FIG. 2 is a schematic view for explaining a drawing reference distancein a diameter decreasing region;

FIG. 3A is a graph showing a relation between a drawing referencedistance and a feed speed in case where 160, 170 and 180 mm diameterglass preforms are drawn into 150 mm target diameter glass rods;

FIG. 3B is a graph showing a relation between a drawing referencedistance and a feed speed in case where 160, 170 and 180 mm diameterglass preforms are drawn into 110 mm target diameter glass rods;

FIG. 3C is a graph showing a relation between a diameter of a glasspreform and a feed speed at a 48 mm drawing reference distance, which isdetermined from data of FIG. 3A;

FIG. 4 is a flow chart showing an example of a drawing process by thecontroller of FIG. 1;

FIG. 5 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Example 1;

FIG. 6 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Comparative example 1; and

FIG. 7 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Comparative example 2.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention will be described below withreference to the attached drawings. FIG. 1 schematically shows anapparatus for fabricating a glass rod according to an embodiment of thepresent invention. The fabricating apparatus has a feeding mechanism 101and a drawing mechanism 110, which are disposed on a frame FR verticallyextending, respectively, a furnace 120 disposed on the frame between thefeeding mechanism 101 and the drawing mechanism 110, and a controller130.

The feeding mechanism 101 has a screw shaft 102 vertically extending androtatably supported, a motor 103 for driving the screw shaft 102, amovable member 104 into which the screw shaft 102 is screwed, and achucking mechanism 105 which is disposed on the movable member 104 andholds a upper end portion of an optical fiber glass preform 201.

The drawing mechanism 110 has a vertically extending and rotatablysupported screw shaft 112, a motor 113 for driving the screw shaft 112,a movable member 114 into which the screw shaft 112 is screwed, and achucking mechanism 115 which is disposed on the movable member 114 andholds a lower end portion of the optical fiber glass preform 201.

The furnace 120 has an annular heater 121 therein, and heats an opticalfiber glass preform 201 passing through a central portion of the heater121.

The controller 130 is constituted by hardware such as a processor and amemory, and required software, and is electrically connected to themotors 103 and 113, and the furnace 120. Specifically, the controller130 controls rotational velocities of the motors 103 and 113, and atemperature in the furnace 120.

Drawing of a glass preform by the apparatus in FIG. 1 will be described.First, a glass preform 201 having a relatively large diameter is fedinto the furnace 120 through the top portion of the furnace by thefeeding mechanism 101. The glass preform 201 fed into the furnace 120 isdrawn from the furnace 120 through a bottom portion of the furnace bythe drawing mechanism 110 so that the glass preform 201 is drawn to arelatively small diameter glass rod. At that time, the feeding mechanism101 and the drawing mechanism 110 are controlled so that a ratio V2/V1between a feed speed V1 and a drawing speed V2 of the glass preform 201becomes a value (D/d)² which is defined by a diameter D of the glasspreform and a target diameter d of the glass rod. That is, a feed speedV1 and a drawing speed V2 are controlled so that a relation defined bythe following formula (1) is satisfied.

V2/V1=(D/d)²  (1)

In the present embodiment, during a drawing control, the diameter D ofthe glass preform for determining a ratio between a feed speed V1 and adrawing speed V2 is acquired from a measured diameter data of the glasspreform, which is obtained by pre-measuring a diameter of the glasspreform before being drawn along a longitudinal direction of the glasspreform before drawing the glass preform. The measured diameter data ofthe glass preform 201 to be drawn is associated with a longitudinalposition of the preform and stored in a storage device such as a memoryin the controller 130. In particular, the diameter D of the glasspreform 201 is acquired from the measured diameter data at a diameteracquisition position which is a predetermined distance from a referenceposition of the heater 121 in the furnace 120. The predetermineddistance is set to a specific drawing reference distance. The specificdrawing reference distance will be discussed later. In addition, a feedspeed V1 is determined so that a feed speed V1 varies depending on afluctuation of the measured diameter data in the longitudinal direction.In particular, a feed speed V1 is determined by using the diameter Dacquired at the diameter acquisition position and diameter and feedspeed data defining a relation between a diameter of the glass preform201 before being drawn and a feed speed V1. The diameter and feed speeddata will be discussed later.

Next, a drawing reference distance will be discussed with reference toFIG. 2. The glass preform 201 is fed into the furnace 120 through thetop portion of the furnace at a feed speed V1, heated by the heater 121,and drawn from the furnace 120 through the bottom portion of thefurnace. Here, a tension is created on the glass preform 201 by settingV2>V1. A heated and softened region of the glass preform 201 isstretched due to a difference between the feed speed and drawing speed,so that a diameter decreasing region 202 where a diameter is graduallyreduced in a longitudinal direction is formed. A diameter of the glasspreform 201 is reduced in the diameter decreasing region 202 thereby arelatively small diameter glass rod 203 is formed.

The glass preform 201 fed into the furnace 120 is heated by the heater202 so that a temperature of the preform 201 in the longitudinaldirection reaches a maximum temperature at a position below a middleposition C of the heater 121 and gradually decreases from the maximumtemperature position downward. Accordingly, a position PM where adeformation rate (an amount of diameter reduction per unit length in thelongitudinal direction) is largest is located below the heater middleposition C at any time.

Here, VM indicates a volume of a glass preform 201 from the heatermiddle position C to the position PM, ND indicates a diameter of theglass preform 201 before being drawn, L indicates a drawing referencedistance. In the present embodiment, a drawing reference distance L isdefined by the following formula (2).

L=VM/(π×(ND/2)²)  (2)

A drawing reference distance L is defined based on a distance from theheater middle position C to the position on the glass preform beingdrawn, and varies depending on a feed speed and a diameter of a glasspreform before being drawn, as described later. That is, a drawingreference distance L is a distance between the heater middle position Cand a specific position which is defined depending on a deformationcondition of the diameter decreasing region on the glass preform.Accordingly, a drawing reference distance L depends on the position PMin the diameter decreasing region 202, where the deformation rate islargest.

In the present embodiment, a diameter before being drawn for adetermination of a feed speed and a drawing speed is acquired from themeasured diameter data at a position away from the heater middleposition C by a predetermined distance. The predetermined distance isset to a specific drawing reference position. In addition, a feed speedv1 is adjusted depending on a diameter fluctuation of the glass preformto be drawn to keep a drawing reference distance at a specific drawingreference distance, thereby a position PM where a deformation rate islargest is maintained at a constant position, so that a glass rod havinga stable and constant diameter can be fabricated. If a drawing referencedistance during drawing was different from an actual specific drawingreference position, the preform is drawn at a different position from aposition corresponding to the actual specific drawing reference distancethereby a finished diameter is fluctuated. Accordingly, if a preformbefore being drawn has a diameter fluctuation in a longitudinaldirection, it is very important to precisely set a drawing referencedistance to the specific drawing reference distance. The diameterdecreasing region 202, however, is located close to the heater 121, soit is difficult to directly measure a shape of the diameter decreasingregion 202 during drawing. For this reason, in the present embodiment, adrawing reference distance during drawing is calculated or estimatedfrom a preliminary experiment or data acquired from a glass performduring being drawn in a steady state.

The present inventor investigated a relation between a drawing referencedistance L, a feed speed V1 and a glass preform diameter D before beingdrawn. In particular, a glass preform diameter D, a glass rod targetdiameter d and a feed speed V1 were set to a variety of values, and adrawing reference distance L was measured from an actual shape of adiameter decreasing region 202. The results are shown in FIGS. 3A to 3C.FIG. 3A shows a relation between a drawing reference distance and a feedspeed in case where 160, 170 and 180 mm diameter glass preforms weredrawn into 150 mm target diameter glass rods, respectively. FIG. 3Bshows a relation between a drawing reference distance and a feed speedin case where 160, 170 and 180 mm diameter glass preforms were drawninto a 110 mm target diameter glass rod. As can be seen from therelation between a feed speed and a glass preform diameter with respectto a plurality of drawing reference distances, a drawing referencedistance L varies depending on a glass preform diameter and a feedspeed. FIG. 3C shows diameter and feed speed data. Specifically, FIG. 3Cshows a relation of a feed speed V1 with respect to a glass preformdiameter D when a drawing reference distance is defined at 48 mm. Thediameter and feed speed data is stored in a storage device such as amemory in the controller 130. Alternatively, a function for defining arelation between a glass preform diameter D and a feed speed V1 can beused instead of the data.

Next, an example of a drawing processing by the above controller will bedescribed with reference to FIG. 4. First, a glass preform 201 having apre-measured diameter is set to the apparatus of FIG. 1, the heater 121is switched on, and the motors 103 and 113 are driven to feed the glasspreform 201 into the furnace 120 (S1).

If starting to heat the glass preform from a drawing start position of ausable region of the preform which can be used for forming a glass rod,a diameter fluctuation may be generated on the drawing start side of thepreform because a temperature distribution of the preform has not beenin a steady state yet. To prevent this, it is necessary to start to heatthe glass preform so as to make a temperature distribution at thedrawing start position steady before a drawing start position of theusable region of the glass preform reaches the heater middle position Cin furnace 120.

A length of a region between a position to start heating the glasspreform and the drawing start position of the usable region of the glasspreform (which is referred to as a preliminary heating region below) ispreferably set to equal to or more than a length of the heater from apoint of view to provide a stable a temperature distribution. Extendingit more than necessary is disadvantageous in production efficiencybecause of a large loss of the glass preform, so it is preferably set toless than three times of the length of the heater.

Next, an acquisition of a current position of the glass preform 201 inthe longitudinal direction relative to the furnace 120 is started (S2).The current position of the glass preform 201 relative to the heater 121can be acquired using, for example, a rotational position detector (notshown in the figures) incorporated in the motor 103. Next, whether thedrawing start position of the glass preform 201 reaches the positionwhere is away from the heater middle position C by a predetermineddistance which is set to a specific drawing reference distance isjudged. If it reaches there, a drawing processing is started (S4).

With the drawing processing being started, a diameter D at the positionPD which is away from the heater middle position C by a predetermineddistance on the glass preform 201 on the move is acquired (S5). Thediameter D can be determined from the current position information ofthe glass preform 201 relative to the heater 121 and a measured diameterdata of the glass preform 201.

Next, a feed speed V1 corresponding to the diameter D acquired at thestep S5 is determined (S6). This can be determined from the diameter andfeed speed data shown in FIG. 3C. There is a diameter fluctuation in alongitudinal direction of the glass preform, and the feed speed V1 isvaried depending on the fluctuation.

Next, a drawing speed V2 is determined (S7). The drawing speed V2 can becalculated from the above formula (1). A control command depending onthe determined feed speed V1 and drawing speed V2 is sent to the motors103 and 113 (S8).

Next, whether the glass preform reaches an ending position of the usableregion is judged (S9). If it does not reach there, the steps S5 to S8are repeated. If it reaches there, the drawing processing is terminated(S10).

In the present embodiment, the diameter and feed speed data as shown inFIG. 3C defines a relation between a feed speed and a glass preformdiameter D before being drawn, so that a drawing reference distance ofthe glass preform during drawing is maintained at a predetermineddistance (a specific drawing reference distance). Accordingly, even ifthe diameter of the glass preform during drawing fluctuates, the feedspeed is adjusted depending on the diameter fluctuation so as to keep adrawing reference distance at the predetermined distance. As a result,the drawing reference distance is maintained at a constant value therebya diameter fluctuation of a drawn glass can be suppressed.

The method of the present embodiment provides a great technical effectespecially in a case where the glass rod target diameter d is 60 to 95%of the glass preform diameter D. As can be seen from a comparisonbetween FIG. 3A and FIG. 3B, an amount of change of drawing referencedistance associated with variation of a feed speed and a diameter isrelatively larger in a drawing process with a relative large diameterreduction ratio in which a 160 mm diameter glass preform is drawn into a150 mm diameter glass rod and the ratio is 94%. Specifically, in a caseof such a relatively large diameter reduction ratio, drawing accordingto the present invention is significantly useful. Also, a change ofdrawing reference distance associated with a variation in a glasspreform diameter can be seen even in the drawing of a relative smalldiameter reduction ratio such as from a 180 mm diameter to a 110 mmdiameter as shown in FIG. 3B, however, the amount of the change isrelatively small. In case of less than 60% of a diameter reduction rate,a conventional method can be alternated, and the method of the presentinvention also can provide a desirable result. On the other hand, incase of over 95% of a diameter reduction rate, it is difficult tomaintain an appropriate drawing load during drawing so that a drawnglass rod may has deflection.

Conventionally, if a diameter fluctuation of glass rod, when thediameter is reduced in a drawing furnace, becomes large, the diameterfluctuation can be modified by re-drawing using an existing glasslathes. However, in a case where a glass rod diameter is more than 110mm, it becomes difficult or impossible to re-draw because heatefficiency decreases during re-drawing using the existing glass lathes.Accordingly, in a case where a glass rod target diameter is over 110 mm,a glass rod having a decreased diameter fluctuation can not befabricated unless the present invention is utilized. In such a case, thepresent invention is effective.

Example 1

A glass preform having tapered portions at both ends, a 1000 mm lengthusable region of the preform, a diameter of 160.5 mm at a drawing startposition, a diameter of 173 mm at a drawing ending position, and afluctuated diameter in the usable region was drawn under conditions of a130 mm heater length, a 48 mm drawing reference distance, a 150 mm glassrod target diameter, and a 200 mm length of the preliminarily heatedregion on the drawing start side. The glass preform was set so that adrawing start position is located above the heater middle position C andspaced from the position C by a distance of 152 mm. The drawing wasstarted at a heater temperature of 2050 degrees Celsius. A feed speed ofthe preliminarily heated region was set to 13.3 mm/min, a largerdiameter portion than 150 mm of the tapered portion on the drawing startside was drawn at a calculated drawing speed using the formula (1) whilea target diameter of the potion was set to the 150 mm glass rod targetdiameter.

A feed amount of the glass preform was set to 0 mm at the time pointthat the 200 mm length of preliminarily heated region passed through theheater middle position C. From here, the glass preform was moved 1000 mmwhich is a length of the usable region. During this time, a feed speedwas determined from the relation between the diameter and feed speed ofthe glass preform of FIG. 3C depending on a diameter fluctuation of theglass preform. A drawing speed was determined by the formula (1) basedon the feed speed, the preform diameter before being drawn and the glassrod target diameter. As a result, a drawing reference distance duringdrawing was kept at the predetermined length (48 mm).

Another 200 mm length region was further drawn, after the usable regionpassed through a position spaced from the heater middle position C by adistance of 48 mm. During this time, a feed speed was maintained at 9.0mm/min, which is a final feed speed of the usable region. The largerdiameter portion than 150 mm of the tapered portion on the drawingending side was drawn at a drawing speed calculated using the formula(1) while a target diameter of the potion was set to the 150 mm. As aresult, as shown in FIG. 5, a very small diameter fluctuation, in whicha difference between a maximum diameter and a minimum diameter isapproximately 1.0 mm, could be obtained.

Comparative Example 1

A glass preform having tapered portions at both ends, a 1000 mm lengthusable region of the preform, a diameter of 160 mm at a drawing startposition, a diameter of 170.5 mm at a drawing ending position, and afluctuated diameter in the usable region was drawn under conditions of a130 mm heater length, a 41 mm drawing reference distance, a 150 mm glassrod target diameter, and a 200 mm length of the preliminarily heatedregion on the drawing start side. A feed speed V1 of the glass preformwas fixed at 10 mm/min, that is, the feed speed V1 is constant. Theglass preform was set so that a drawing start position is located abovethe heater middle position C and spaced from the position C by adistance of 159 mm. Drawing was started at a heater temperature of 2050degrees Celsius. A larger diameter portion than 150 mm of the taperedportion on the drawing start side was drawn at a drawing speedcalculated using the formula (1) while a target diameter of the potionwas set to the 150 mm glass rod target diameter. A feed amount of theglass preform is set to 0 mm at the time point that the 200 mm length ofpreliminarily heated region passed through the heater middle position C,and the glass preform was fed by 1000 mm length of the usable region.During this drawing, a drawing speed is determined by the formula (1)based on the feed speed and diameter of the glass rod.

Another 200 mm length region was further drawn, after the usable regionpassed through a position spaced from the heater middle position C by adistance of 41 mm. A larger diameter portion than 150 mm of the taperedportion on the drawing ending side was drawn at a drawing speedcalculated using the formula (1) while a target diameter of the potionwas set to the 150 mm glass rod target diameter. As a result, as shownin FIG. 6, a range width of diameter fluctuation was approximately 3.1mm, and the diameter fluctuation was larger than that of Example 1.

Comparative Example 2

A glass preform having tapered portions at both ends, a 1000 mm lengthusable region of the preform, a diameter of 161.2 mm at a drawing startposition, a diameter of 169 mm at a drawing ending position, and afluctuated diameter in the usable region was drawn under conditions of a130 mm heater length, a 48 mm drawing reference distance, a 150 mm glassrod target diameter, and without a preliminarily heated region on thedrawing start side. The glass preform was set so that a drawing startposition is located below the heater middle position C and spaced fromthe position C by a distance of 48 mm. Drawing was started at a heatertemperature of 2050 degrees Celsius.

A feed amount of the glass preform was set to 0 mm when the glasspreform was initially set. From here, the glass preform was moved 1000mm which is the length of the usable region. During this time, a feedspeed was determined from the relation between the diameter and feedspeed of the glass preform of FIG. 3C depending on a diameterfluctuation of the glass preform. A drawing speed was determined by theformula (1) based on the feed speed, the preform diameter before beingdrawn and the glass rod target diameter. Another 200 mm length regionwas further drawn, after the usable region passed through a positionaway from the heater middle position C by a distance of 48 mm. Duringthis time, a feed speed was maintained at 10.1 mm/min, which is a finalfeed speed of the usable region. The larger diameter portion than 150 mmof the tapered portion on the drawing ending side was drawn at a drawingspeed calculated using the formula (1) while a target diameter of thepotion was set to the 150 mm. As a result, as shown in FIG. 7, a largediameter fluctuation which is +1.5 mm/−3 mm was generated on the drawingstart side, and the diameter fluctuation was larger than that of Example1.

INDUSTRIAL APPLICABILITY

According to the present invention, a diameter fluctuation of a drawnglass rod in a longitudinal direction can be suppressed when drawing ata relatively large diameter reduction ratio.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A method of fabricating a glass rod by feeding arelatively large diameter glass preform into a furnace through a topportion of the furnace and drawing the glass preform from the furnacethrough a bottom portion of the furnace so that the relatively largediameter glass preform is drawn into a relatively small diameter glassrod, comprising the steps of: controlling a feed speed (V1) and adrawing speed (V2) of the glass preform so that a ratio (V2/V1) betweenthe feed speed (V1) and the drawing speed (V2) becomes a value ((D/d)²)determined based on a diameter (D) of the glass preform and a targetdiameter (d) of the glass rod; acquiring the diameter (D) of the glasspreform for determining the ratio from a measured diameter data, themeasured diameter data being obtained by measuring a diameter of theglass preform before being drawn along a longitudinal direction of theglass preform; and determining the feed speed (V1) so that the feedspeed (V1) varies depending on a fluctuation of the measured diameterdata in the longitudinal direction.
 2. The method of claim 1, whereinthe step of acquiring the diameter (D) of the glass preform comprisesacquiring a diameter at a diameter acquisition position on the glasspreform before being drawn from the measured diameter data, the diameteracquisition position being away from a reference position of the furnaceby a predetermined distance (L) on the glass preform and within adiameter decreasing region of the glass preform.
 3. The method of claim2, wherein the step of determining the feed speed (V1) comprisesdetermining the feed speed (V1) using the diameter (D) acquired at thediameter acquisition position and diameter and feed speed data defininga relation between a diameter of the glass preform and the feed speed(V1).
 4. The method of claim 3, wherein the diameter and feed speed datadefines the relation so as to decrease the feed speed in accordance withan increase of a diameter of the glass preform and increase a feed speedin accordance with a decrease of a diameter of the glass preform.
 5. Themethod of claim 4, wherein the diameter and feed speed data defines therelation so that a drawing reference distance of the glass preformduring drawing is maintained at the predetermined distance, wherein thedrawing reference distance is defined as a distance between thereference position of the furnace and a specific position defineddepending on a deformation condition of the diameter decreasing region,and varies depending on the diameter before being drawn or the feedspeed of the glass preform.
 6. The method of claim 5, wherein an amountof change of the drawing reference distance with respect to a variationof the diameter before drawn or the feed speed of the glass preformdiffers depending on a diameter reduction ratio, the diameter reductionratio being defined as a ratio (d/D) between the diameter (D) beforedrawn and the target diameter (d).
 7. The method of claim 6, wherein thediameter reduction ratio is within 60 to 95%.
 8. The method of claim 7,wherein the target diameter (d) of the glass rod is more than or equalto 110 mm.
 9. The method of claim 1, further comprising: starting toheat the glass preform before a drawing starting position of a usableregion in the glass preform reaches the reference position of thefurnace, wherein a distance between a position for starting to heat theglass preform and the drawing starting position of the usable region isset to equal to or more than a length of a heater in the furnace in alongitudinal direction and less than three times of the length.
 10. Anapparatus for fabricating a glass rod, comprising: a furnace; a feedingmechanism configured to feed a relatively large diameter glass preforminto a furnace through a top portion of the furnace; a drawing mechanismconfigured to draw the glass preform from the furnace through a bottomportion of the furnace so that the relatively large diameter glasspreform is drawn into a relatively small diameter glass rod; acontroller configured to control a feed speed (V1) of the glass preformby the feeding mechanism and a drawing speed (V2) of the preform by thedrawing mechanism so that a ratio (V2/V1) between the feed speed (V1)and the drawing speed (V2) becomes a value ((D/d)²) determined based ona diameter (D) of the glass preform and a target diameter (d) of theglass rod, wherein the controller comprises: an acquisition unitconfigured to acquire the diameter (D) of the glass preform fordetermining the ratio from a measured diameter data, the measureddiameter data being obtained by measuring a diameter of the glasspreform before being drawn along a longitudinal direction of the glasspreform; and a determination unit configured to determine the feed speed(V1) so that the feed speed (V1) varies depending on a fluctuation ofthe measured diameter data in the longitudinal direction.